Modem propeller blades typically include root portions which extend into the hub arm of the hub of the propeller system and which are secured to and rotatable relative to the hub arm via a retention assembly. Typically the retention assembly includes one or a plurality of ball bearing assemblies which permit the rotation of the blade in the hub arm for accomplishing pitch change of the blade for altering the speed of the propeller and accordingly, the aircraft. Typical propeller blade systems using this type of arrangement include the system shown in U.S. Pat. Nos. 3,637,323, 2,248,590, 4,921,403 and 5,415,527. As can be seen from the drawings of each of these systems, the root of the propeller blade extends into the hub arm and thereby is connected to the inner surface of the hub arm via a retention mechanism, typically in the form of ball bearing assemblies and fastening elements. As can be seen from observing these figures, the removal of the blade from the hub arm for maintenance, repair or for other reasons, is a cumbersome task, involving the breakdown and disassembly of the retention assembly as well as the draining of the lubricating fluids. Accordingly, while such systems offer advantages with regard to blade preload, and blade retention, in addition to the above, the root portion of the blade is typically complex in design since it must be formed to have its outer surface interact with the retention mechanism.
The prior art does include blades of the bolt-on variety which have been designed, in part, to defeat the shortcomings of the aforementioned designs. Several systems of this variety are shown in U.S. Pat. No. 5,074,754, directed to rotor blades, and U.S. Pat. No. 3,734,642, , which is directed to a helicopter or aircraft rotor or blade. While the blade of U.S. Pat. No. 5,074,754 is bolted on via the application of retention plate 16 fastened to retention member 12 with blade 10 locked therebetween, this design suffers from various defects. For one, the blade is not directly retained by the fastening means or fasteners 18, but is indirectly retained via retention plate 16. In this manner, a level of protection is lost since failure of the plate as opposed to failure of multiple fasteners can lead to blade failure. In addition, the blade shown in this patent is apparently not a composite blade typical of current propeller systems and thereby has the disadvantages associated with not being formed from composite.
The blade in U.S. Pat. No. 3,734,642 is formed from a spar 18, 18' a primary outer sleeve 24 and a secondary inner sleeve or cuff 38 located inwardly of the spar. The spar in this case is formed from a composite material, but as shown in the FIG. 1 and FIG. 2 embodiment, the spar 18 or 18' does not extend to the base portion of the blade for fastening to the retention assembly or hub arm 14 since the composite, at the time of this patent, was not able to withstand the compressive forces of the fastening mechanisms. This is indicated in the background section where the author points out that the composite material cannot be bolted directly to the hub or other parts thereof because due to its fibrous nature, the composite material has a low shear strength at the bolt holes. Thereby, a rupture of the bonding connection between the composite blade or spar and the metallic cuff could progress so rapidly that adequate warning of impending blade failure could not be provided to the pilot in order for him to land the aircraft. Accordingly, the inventor for this patent set out to form a composite blade with an outer and inner sleeve for supporting the composite spar at the fastening interface. However, since the composite spar 18 does not extend to the base portion of the blade and accordingly is not directly fastened to the hub via fasteners 28, a level of security of the blade is lost. Reliability of securement is based solely on an adhesive holding the primary sleeve and cuff to the composite spar and thus, the composite spar to the propeller.
There exists a need, therefore, for a bolted-on composite propeller blade, which propeller blade is primarily formed from a composite spar, wherein the composite spar forms the elongated and base portion of the composite blade such that the blade is fastened to the retention assembly or hub arm by fastening the composite spar to the retention assembly or hub arm, thereby creating a strong, safe and easily removable composite propeller blade which does not disturb any preload of the retention assembly thereof.